What does Luke 3:12 mean?

Those who heard John the Baptist were commanded to repent—to run from sin—and to live lives consistent with that repentance (Luke 3:7–9). Those who accepted this message were baptized as a public expression of their decision. All this was part of John's role as the herald of Messiah (Luke 3:1–6). This passage shows John's response to questions from various groups about how to live out that repentance. Each corresponds to some temptation common to that group. For people in general, John's advice is to use one's abundance to bless those who are in need (Luke 3:10–11).

Here, John is approached by tax collectors. This was an era of Roman occupation: the Jewish people were subjects of a pagan empire. Locals were hired by the invaders to collect and submit taxes. This was doubly offensive to the people of Israel. Not only did tax collectors work on behalf of their nation's oppressors, but they also interacted with unclean Gentiles. As a result, the term "tax collectors" was often used as general metaphor for distasteful, immoral people (Mark 2:16; Matthew 5:46).

Worse, many tax collectors were prone to corruption. Romans cared little how much money was taken from conquered people, so long as the correct amount was given to imperial leaders. Unscrupulous tax collectors could add extravagant amounts to the taxes they collected, knowing they could threaten others with arrest or imprisonment. This is the sin which John will address when giving them advice in the next verse (Luke 3:13).